Publications and other reference materials referred to herein are numerically referenced in the following text and respectively grouped in the appended Bibliography which immediately precedes the claims.
Surface Plasmon Resonance (SPR) is a quantum electromagnetic (EM) phenomenon arising from the interaction of light with free electrons at a metal-dielectric interface emerging as a longitudinal EM wave in a two dimensional gas of charged particles such as free electrons in metals. Under certain conditions the energy carried by the photons is transferred to collective excitations of free electrons, called surface plasmons (SPs), at that interface. This transfer of energy occurs only at a specific resonance wavelength of light when the momentum of the photon matches that of the plasmon[1]. The SPs excited are strongly localized across the interface, and may be considered classically as EM surface waves that propagate along the interface and decay exponentially with distance normal to the interface. SPR is responsible for a dip in reflectance at the specific wavelength, the dip resulting from the absorption of optical energy in the metal. Since SP waves are tightly bound to metal-dielectric interfaces penetrating around 10 nm into the metal (the so-called skin-depth) and typically more than 100 nm into the dielectric (depending on the wavelength), they concentrate EM wave in a region that is considerably smaller than their wavelength.
There are two main types of SPs with respect to their propagation characteristics along the interface: extended or propagating and localized. The propagating SP is considered as more classical since it has been known for a longer time. However the latest advancements in nanotechnology have made the fabrication of structures with nm scale features feasible, thus the localized SPR has become a subject of immense interest during the last two decades. Localized SPs are excited in metallic structures with lateral dimensions less than half the wavelength of the exciting EM wave. A third type of plasmon may be mentioned called long range SPR (LRSPR) which exists in thin metal films or strips characterized by low attenuation and travelling along the surface for distances up to few mm in the visible or even a few cm in the infrared. This latter type might have applications in active photonic components and highly sensitive sensors particularly of large biological entities such as cells.
In the case of propagating SPR (PSPR), plasmons propagate along the interface between metal and dielectric for distances on the order of microns to tens and even hundreds of microns and decay evanescently in the z direction (see FIGS. 1a to 1f normal to the interface with 1/e decay length on the order of half the wavelength (˜200 nm for wavelengths in the visible range). The interaction between the metal surface-confined EM waves and the molecular layer of interest leads to shifts in the plasmon resonance, which can be observed in three main modes: (a) angle resolved mode, (b) wavelength shift mode, and (c) imaging mode. In the first two modes, one measures the reflectivity of light from the metal surface as a function of either wavelength (at constant incidence angle) or as a function of incidence angle (at constant wavelength). The third mode uses light of both constant wavelength and incidence angle to interrogate a two-dimensional region of the sample, mapping the reflectivity of the sample as a function of position. In each of these modes one can measure intensity, phase or polarization change.
PSPR biosensors have been widely applied in a diverse range of fields, including molecular recognition, and disease immunoassays, etc. Even though conventional SPR biosensors are more sensitive than other label-free devices, they are still unable to achieve the direct detection of small molecular (few hundreds of Daltons) interactions or low molecular concentrations (physiological concentration) on the surface of the biosensor. Consequently, various proposals have been developed to enhance the sensitivity or resolution of biosensors by using different SPR modes or detection methods. Also, various localized SPR (LSPR) biosensors have been proposed which employ the strong UV-Vis absorption band of the metal nanoparticles to yield an area mass detection limit of 100-1000 pg/mm2. However, this detection capability is poorer than that of conventional PSPR biosensors by an order of 10-100 times.
FIG. 1a to FIG. 1f schematically show several techniques for enhancing the wave vector to excite the SP wave. FIG. 1a shows prism coupling on the top of the metallic film, in which the wave vector is enhanced by the prism refractive index. This method is known as the Kretschmann configuration. In FIG. 1a are shown the incident light 2, reflected light 4, metal film 6, analyte 32, and prism 10. The field distribution is symbolically shown as 12.
FIG. 1b shows prism coupling with a thin gap (filled with analyte) between bulk metal and the prism in what is known as the Otto configuration. In this configuration the wave vector is enhanced by the prism refractive index and the coupling occurs via evanescent waves since the air gap is thinner than the light penetration depth. In FIG. 1b are shown the incident light 2, reflected light 4, air gap 14, bulk metal 16, prism 10 and the field distribution 12.
FIG. 1c shows coupling through a diffraction grating, in which the wave vector is enhanced by the diffraction. In FIG. 1c are shown incident light 2, reflected light 4, metal film 6, analyte 32, the field distribution 12 and diffraction grating 26.
FIG. 1d shows waveguide coupling. In FIG. 1d are shown metal film 6, analyte 32, the field distribution 12, waveguide 18, and the transmitted light 20.
FIG. 1e shows fiber coupling. Shown in this figure are metal film 6, analyte 32, and fiber 22. FIG. 1f shows nano probe coupling. Shown in this figure are bulk metal 16, the field distribution 12, and a scanning near field optical microscope (SNOM) probe 24. In FIG. 1E and FIG. 1F the coupling mechanism is based on the evanescent waves either within a waveguide interface or in the near field of a scanning microscope.
Perturbation in the substrate refractive index (dielectric 32) causes a change in the incident light wave vector and consequently a shift in the resonance wavelength for a fixed incidence angle or a shift in the incidence angle for a fixed wavelength because the wave vector along the interface is:kx=2πnp sin θp/λ.
The excitation of plasmons by transverse magnetic (TM) polarized coherent light in the KR configuration requires a prism [1], which matches between the wave vector of the incidence light along the interface kx=npk0 sin θp, where k0=2π/λ and the k vector of the surface plasmon ksp=k0(εmεa(εm+εa))1/2:
                                                        ɛ              p                                ⁢          sin          ⁢                                          ⁢                      θ            p                          =                  Re          ⁢                      {                                                                                ɛ                    m                                    ⁢                                      ɛ                    a                                                                                        ɛ                    m                                    +                                      ɛ                    a                                                                        }                                              (        1        )            
When εp is the dielectric constant of the prism, θp is defined as the propagation angle in the prism:
            k      x        =                            ɛ          p                    ⁢              ω        c            ⁢      sin      ⁢                          ⁢              θ        p              ,where c is the velocity of light in free space and ω is the radiant frequency. εm and εa are the complex dielectric constants of the metal and dielectric (analyte) respectively. According to equation (1) the SP can be excited at a specific angle depending on the light wavelength through the materials dispersion relation. The most popular SPR sensing scheme uses the prism coupling in the Kretschmann-Raether (KR) arrangement (FIG. 1A). In KR configuration, reflectivity is measured as a function of angle of incidence—called angular modulation (AM)—or wavelength—called wavelength modulation (WM). In AM a single wavelength, usually a collimated laser beam, is incident on the metal film through the prism while scanning through different incidence angles. The SPR dip is observed in the reflectivity versus incidence angle spectrum of a collimated beam. Although AM uses a single frequency and a collimated beam, the required scanning is problematic in particular when high accuracy and fast speed are required. Using an imaging scheme, the possibility of detecting arrays or imaging surfaces with very low contrast was demonstrated [2]. In these publications [2] however the incident beam is usually a parallel beam focused (or non-focused) on the metal-analyte surface and the surface is imaged with a lens and camera. In a less known configuration a converging [3] or diverging circular beam [4] is used which contains a wide range of angles and the divergent output beam falls on a detector array so that the SPR signal of reflectivity versus angle is obtained in a parallel manner without the need for scanning. Recently the research group led by the present inventor has published two papers [5, 6] on a SPR imaging method using diverging circular beams and a fast line detection algorithm called the Radon transform. However the disadvantages of the circular diverging beam approach are that the circular beam contains many spatial components that cannot excite an SP wave and their polarization vector contains both TE and TM components. As a result the contrast of the SPR is deteriorated, it is impossible to perform phase imaging, and multichannel imaging is more problematic.
Another important factor affecting the sensitivity is the penetration depth of the electromagnetic field inside the analyte which can be estimated from the following equation:
                              δ          a                =                              λ                          2              ⁢              π                                ·                                                                      ɛ                  a                                +                                  ɛ                  mr                                                            -                                  ɛ                  a                  2                                                                                        (        2        )            
For example for silver at λ=1500 nm, εmr=−115.5, which is much larger than εa=1.769, thus giving a penetration depth of about δa=0.96λ≈1.44 μm. For the visible range λ=600 nm, on the other hand εmr=−14.14, thus giving δa=0.316λ≈0.19 μm. Hence the penetration depth in the near IR (NIR) range is larger by a factor of 8 than that in the visible range, although the wavelength ratio is only 2.5.
The propagation length Lx of the SP along the surface of the metal at the interface with the analyte can be estimated from:
                              L          x                =                              λ                          2              ⁢              π                                ·                                    ɛ              mr              2                                      ɛ              mi                                ·                                    [                                                                    ɛ                    a                                    +                                      ɛ                    mr                                                                                        ɛ                    a                                    ·                                      ɛ                    mr                                                              ]                                      3              /              2                                                          (        3        )            
The imaginary part of silver dielectric constant at 1500 nm is εmi=12.3 and hence Lx≈72λ108 μm. For the visible (λ=600 nm) on the other hand εmi=0.96 giving Lx≈11.5λ≈6.9 μm. Hence the plasmons in the IR region travel a longer range along the interface, an important fact for enhancing the sensitivity and improving the detection limit. In order to improve the propagation length and the penetration depth further the long range SPR (LRSPR) configuration is introduced in which an additional dielectric layer buried underneath the metal layer is added with refractive index close to that of the analyte. In this mode two SPR dips are obtained where one is excited at the buried layer-metal interface and one at the metal-analyte interface. The former is not sensitive to the variations in the analyte refractive index and therefore can be used as a reference dip to compensate for temperature fluctuations or other system noise effects. The latter dip however exhibits enhanced sensitivity to the analyte refractive index changes due to the increase in the penetration depth and the propagation length.
It is a purpose of the present invention to provide SPR sensors that have an enhanced figure of merit and lower limit of detection than prior art SPR sensors.
Further purposes and advantages of this invention will appear as the description proceeds.